The multi-layer glass is applied to, for example, a window glass for building materials which requires high thermal insulation performance, a door of an industrial refrigerator or freezer, and an opening portion of window material for a transport equipment such as an automobile which requires energy saving. In recent years, along with demand for the window glass having excellent thermal insulation performance, the multi-layer glass having thermal insulation performance has been frequently used and rapidly spread.
As the multi-layer glass, there is a multi-layer glass panel in which a space formed by opposing plate glasses is filled with air or rare gas such as argon, or a vacuum insulation multi-layer glass panel in which the space is vacuum evacuated.
In the vacuum insulation multi-layer glass, in order to hermetically seal the space (hereinafter, referred to as a gap) formed by the opposing plate glasses, low melting point glass having a low gas permeability is used in a vacuum sealing portion. During vacuum sealing, spacers are arranged at equal intervals in the gap and a distance between the glass panels is maintained at a thickness of about 0.2 mm so that the glass panel is not broken by collapse of the gap due to a pressure difference from atmospheric pressure. Further, in order to improve the thermal insulation performance, there is also a multi-layer glass provided with two thermal insulation layers of a vacuum layer and a rare gas-filled layer. However, the rare gas-filled layer requires a thickness of about 10 mm of the thermal insulation layer, and a thickness of the entire multi-layer glass is about 20 mm.
The vacuum insulation multi-layer glass is generally produced by vacuum evacuating the gap in the panel by using an evacuation pipe.
In Patent Document 1, at least one hole through the plate glass is provided, the evacuation pipe made of glass or metal is connected to the through-hole, the evacuation pipe is connected to a vacuum pump or the like, and the gap is vacuum evacuated. After evacuation, an evacuation opening portion is sealed with a cap or the like made of glass or metal.
Patent Document 2 discloses a technology for producing a plasma display panel by evacuating and sealing the gap in the glass panel in a vacuum environment without using the evacuation pipe. By this consistent vacuum production without exposure to the atmosphere, the vacuum sealing can be carried out without degrading properties of members in the plasma display panel.
Patent Document 3 discloses production of the multi-layer glass by sealing the evacuation pipe during evacuation through the evacuation pipe by using a partition wall provided with a slit, and then by cutting out a portion which does not include the evacuation pipe.